Subcortical and neocortical mechanisms of visual-somatosensory interplay.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see The concept of unisensory cortices has been challenged by growing evidence that neurons in primary sensory regions respond to stimuli of different modalities (Driver and Noesselt, 2008). The traditional view is that these crossmodal influences result from backward projections from higher-order areas to primary cortical areas. This feedback hypothesis is supported by a large body of evidence showing cross-modal attentional modulation in lower-order sensory regions (Driver and Noesselt, 2008). More recently, it has been proposed that feedforward and lateral pathways at the earliest processing stages also mediate mul-tisensory interactions. This view is based on two lines of evidence. First, multisensory interactions in sensory cortices first appear 30 –50 ms after stimulus onset. Thus, they are faster than those resulting from feedback connections. Second, anatomical studies reveal direct connections between primary sensory cortices in nonprimate animals (Schroeder and Foxe, 2005). The impact of lateral cortical connectivity on multisensory interactions is unclear. A recent report by Sieben et al. (2013) provided anatomical and electrophysio-logical evidence for lower-order interactions between primary visual and somatosensory cortex (V1 and S1). The authors extracellularly recorded electrical activity evoked by unimodal visual (light flashes) or tactile stimulation (whisker deflection) from multiple sites in V1 and S1. Unimodal stimulation was compared with bimodal stimulation consisting of flash and whisker deflection simultaneously presented in the same (congruent) or in opposite (incongruent) hemifields. To investigate whether the visual cortex mediates visual-somatosensory interactions , the authors blocked action potentials in V1 by intracortical injection of lidocaine, and studied the effect of this block on so-matosensory evoked potentials, as well as stimulus-induced and spontaneous oscillations in S1. Finally, anatomical tracing of neurons in S1 and V1 was performed postmortem. Extracellular recordings from all layers of the barrel field in S1 revealed that bi-modal stimulation enhanced the early components of evoked potentials, compared with unimodal stimulation. In the rat, evoked potentials are characterized by a first large positive component (P1), followed within 10 ms by a negative component (N1), and then by two slower components (N2 and P2). Congruent bimodal stimulation elicited a superadditive augmentation of the P1 and N1 components of evoked …